Neutron spectrometry with spherical hydrogen proportional detectors

Spherical hydrogen proportional detectors (HPD) of 40 mm diameter with different pressures are used in neutron spectrometry in energy range 20-1200 keV. This spherical shape detector having anode wire along one diameter was designed to give a response to neutrons which is independent to their direction of travel with respect to the counter axis (anode). Nevertheless, in practice some imperfectness of spherical detector response exists. The reactor iron filtered beam was assembled with the aim to study detector response dependent on the angle (90° and 0°; perpendicular and parallel) between neutron beam and detector anode. It was confirmed that the dominant effect is caused by neutron shielding properties of the massive end-parts of detector (insulators, aluminum ending). This effect produces 7-13% decrease of registered neutrons depending on energy interval. The resonance energy structure with characteristic maxima of iron filtered beam is used for energy calibration. Comparison with He-3 admixture calibration method is presented. Negative energy shift of measured maxima in neutron beam spectrum is observed. Energy shift depends on high voltage, on the angle between neutron and detector anode and on energy of neutron peak. This shift effect is caused by gas multiplication saturation close to anode wire.